High intensity discharge (HID) lamps include mercury vapor, metal halide, high and low pressure sodium, and xenon short-arc lamps. HID lamps produce light by generating an electric arc across two spaced-apart electrodes housed inside a sealed quartz or alumina arc tube filed with gas or a mixture of gas and metals. The arc tube is typically filled under pressure with pure xenon, a mixture of xenon-mercury, sodium-neon-argon, sodium-mercury-neon-argon, or some other mixture such as argon, mercury and one or more metal halide salts. A metal halide salt (or metal halide) is a compound of a metal and a halide, such as bromine, chlorine, or iodine. Some of the metals that have been used in metal halide lamps or bulbs include indium, scandium and sodium. Xenon, argon and neon gases are used because they are easily ionized, produce some level of immediate light, and facilitate the striking of the arc across the two electrodes when voltage is first applied to the lamp. The heat generated by the arc then vaporizes the sodium, mercury and/or metal halides, which produce light as the temperature and pressure inside the arc tube increases.
A pure xenon short-arc lamp produces a very white light (a correlated color temperature of about 6420 K) with about 10% of the total emitted light in the near infrared (850 to 900 nm). Xenon-mercury lamps produce a more bluish-white light. All xenon short-arc lamps generate significant amounts of ultraviolet radiation. Mercury vapor-based lamps produce a bluish light, but can be color corrected by coating the inside of a glass bulb placed around the arc tube with phosphor, which converts some portion of the ultraviolet light generated by the light into red light. Mercury vapor-based lamps produce significant ultraviolet (UV) radiation, even when protective measures are taken to block some of the UV radiation. Sodium-based lights generally produce an orange/yellow to pink/orange light, but with higher pressures within the arc tube can produce a whiter light (having a color temperature of around 2700 K). By altering the mixture of metal halides in a metal halide lamp, it is possible to generate light with varying levels of intensity and correlated color temperatures as low as 3000 K (very yellow) to as high as 20000 K (very blue). The color temperature of the sun is measured at 5770 Kelvin (K), with daylight ranging from about 5000 to 6500 K.
Since HID lamps are negative resistance devices, they require an electrical ballast to provide a positive resistance or reactance that regulates the arc current flow and delivers the proper voltage to the arc. Some HID lamps, called “probe start” lamps, include a third electrode within the arc tube that initiates the arc when the lamp is first lit. A “pulse start” lamp uses a starting circuit referred to as an igniter, in place of the third electrode, that generates a high-voltage pulse to the electrodes to start the arc. Initially, the amount of current required to heat and excite the gases is high. Once the chemistry is at its “steady-state” operating condition, much less power is required, making HID lamps more efficient (producing more light with less energy over a long period of time) than filament based lights.
The majority of light generated by a short gap HID lamp is produced by a small line source of plasma. This relatively small light source enables the output of the HID lamp to be more easily focused into an intense, narrow beam than many other light sources. A concave (parabolic or elliptical) shaped reflector, with a hole in the bottom through which the HID lamp is inserted, is used to focus the light. Most reflectors are formed from polished aluminum, which is sometimes coated with other reflective materials. To the naked eye, the surface of the reflector looks very smooth and highly reflective, but upon closer inspection, the surface of most reflectors is covered with irregularly shaped jagged ridges and valleys, left by the forming process, that inefficiently reflect light. An uneven surface can result in light of different wavelengths being refracted on the surface of the reflector, instead of being properly focused into a defined beam, or distribution pattern. This refracted light will reduce the efficiency of the system by creating more “stray” light rays (with less of the light generated by the HID lamp making it into the desired light beam or light distribution pattern). Accordingly, a better prepared and processed reflector can achieve greater efficiency as an electro-optical system.
A smaller arc gap spacing between the lamp's electrodes will produce a smaller arc and a smaller line source, which can, in turn, be even more narrowly focused into an intense beam of light by an appropriate reflector. This makes HID lamps ideal for lighting applications that require a beam of light that can travel great lengths to clearly illuminate distant objects, such as search lights, targeting lights, flash lights and other security, rescue, police and military applications. HID lamps could also be useful in police and military applications where an extremely intense light is used to temporarily blind and disorient a person. When used as a non-lethal weapon, it is very important that the HID lamp produce little UV radiation, or that most of the UV radiation generated by the lamp be filtered out, so the retinas of the person subjected to the beam of light generated by the HID lamp will not be damaged.
While it is important to limit UV radiation produced by an HID lamp, it can also be important to limit visible light and to generate, and not excessively limit, the infrared light produced. Infrared light is often used in covert military operations to enhance the effectiveness of night vision goggles. Since it is not always possible or preferable to equip a vehicle, craft or person with different lighting sources for visible and infrared light, such as during covert military operations where the weight carried by an individual needs to be kept to a minimum, it is sometimes necessary to apply a filter to a single HID lamp light (a HID light) so as to block visible light while continuing to pass near infrared and infrared light. If the HID light is to be used in covert situations, it is critically important that the filter block as much visible light as possible in order to prevent the user of the HID light from being detected.
Filtering visible light from the intense beam of light generated by an HID lamp is much more difficult than filtering more diffuse light sources. For example, a red absorption glass filter rated to block all light below 750 nm (the upper limit of the visible light spectrum), might still allow some amount of visible light from a HID lamp through the filter. Even stronger filters, on the other hand, might block all light, including the infrared light, or cut back so far on the infrared light as to reduce the usefulness of the light source. For example, in covert military operations, a high intensity infrared illuminator may be necessary to improve the effectiveness of night vision goggles. This is especially true for Generation III night vision goggles used by the U.S. military and Allied Forces that utilize image intensification (I2) technology to intensify ambient light.
The peak performance, or radiant sensitivity, of the gallium-arsenide photocathode utilized in Generation III systems is within the 450 to 950 nm region of the spectrum. Unfortunately, many allegedly covert infrared illuminators utilize intense filters that either block the majority of light transmission in the 700 to 1000 nm range, or block all light transmission below 875 nm and a large percentage of light transmission up to 900 nm, thereby limiting the illuminator to either the narrow band between 900 to 950 nm, or generating little to no useable illumination at all. Accordingly, a covert operation filter is needed that will work with a highly efficient HID light and reflector assembly to block all visible light transmission below 800 nm, block some large portion of light in the 800 to 860 nm wavelength range, and reflections of other light from the outer surface of the filter, while maximizing the transmission of infrared light in the range most useable for illumination by Generation III night vision systems.